docs: add Astro integration guide (#579)

Add comprehensive Astro integration documentation covering:
- Setup and environment configuration
- API client implementation
- Content fetching patterns
- Dynamic routes with SSG/SSR/hybrid modes
- Pagination
- Deployment to Cloudflare
- Troubleshooting guide

Creates new Integrations section in navigation.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: lane711